This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:                3GPP third generation partnership project        BSR buffer status report        BW bandwidth        CQI channel quality indicator        CRC cyclic redundancy code        DCI downlink control information        DL downlink (eNB towards UE)        DRX discontinuous reception        DTX discontinuous transmission        eNB E-UTRAN Node B (evolved Node B)        EPC evolved packet core        E-UTRAN evolved UTRAN (LTE)        FDD frequency division duplex        FDMA frequency division multiple access        LTE long term evolution        MAC medium access control        MBSFN multicast/broadcast single frequency network        MM/MME mobility management/mobility management entity        Node B base station        O&M operations and maintenance        OFDMA orthogonal frequency division multiple access        PDCCH physical downlink control channel        PDCP packet data convergence protocol        PDSCH physical downlink shared channel        PHY physical        PMI precoding matrix index        PUSCH physical uplink shared channel        RACH random access channel        RI rank information        RLC radio link control        RRC radio resource control        SC-FDMA single carrier, frequency division multiple access        SGW serving gateway        SR scheduling request        TA timing advance        TDD time division duplex        UE user equipment        UL uplink (UE towards eNB)        UTRAN universal terrestrial radio access network        
The specification of a communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRANLTE or as E-UTRA) is currently nearing completion within the 3GPP. As specified the DL access technique is OFDMA, and the UL access technique is SC-FDMA.
One specification of interest is 3GPP TS 36.300, V8.6.0 (2008-09), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 8).
FIG. 1 reproduces FIG. 4.1 of 3GPP TS 36.300, and shows the overall architecture of the E-UTRAN system. The E-UTRAN system includes eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE (not shown). The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME (Mobility Management Entity) by means of a S1 MME interface and to a Serving Gateway (SGW) by means of a S1 interface. The S1 interface supports a many-to-many relationship between MMEs/S-GW and eNBs.
The eNB hosts the following functions:                functions for RRM: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both UL and DL (scheduling);        IP header compression and encryption of the user data stream;        selection of a MME at UE attachment;        routing of User Plane data towards the Serving Gateway;        scheduling and transmission of paging messages (originated from the MME);        scheduling and transmission of broadcast information (originated from the MME or O&M); and        a measurement and measurement reporting configuration for mobility and scheduling.        
It has been agreed in 3GPP that in LTE the UE receives downlink assignments and uplink grants on the PDCCH. The UE attempts to blindly decode the received PDCCHs in every subframe by checking the CRC masked with an identifier (id) known to the UE. Typically the UE does not know in advance when it will receive downlink assignments addressed to it. However, the situation is different regarding uplink data transmission. Although an uplink grant for data transmission is made by the eNB, making an UL grant to the UE is subject to the UE first having indicated that it has data pending in a buffer that is waiting to be transmitted. For this reason it can be appreciated that the UE will know when and when not to expect to receive an UL grant specifically for data transmission, and thus in principle the UE may avoid looking for an UL grant when one is not required.
However, it has also been agreed in 3GPP RAN WG #1 that the eNodeB can in any subframe (excluding subframes when the UE is configured for DRX/DTX) force the UE to send an aperiodic CQI report. The aperiodic CQI request is triggered with one specific bit in the UL grant. Furthermore, it is possible to request the aperiodic CQI report transmission without any simultaneous UL data transmission, e.g., the UE would transmit only the aperiodic CQI report (while possibly having no UL data buffered for transmission).
As presently specified each UE is configured via RRC signaling to one aperiodic CQI reporting mode (before explicit configuration a default mode is assumed depending on the transmission mode) and, as a result, the UE is required to monitor the PDCCH for UL grants even when no other UL transmission is needed by the UE.
In order to be able to report the CQI/PMI/RI, the UE needs to first perform the measurement to determine the instantaneous channel quality. In LTE Rel-8, it has been agreed that if the CQI/PMI/RI report shall be transmitted in the UL subframe n, the DL subframe where the CQI/PMI/RI measurements are performed (also known as the CQI measurement reference period or CQI reference resource) is the DL subframe n-4. The principle is illustrated in the FIG. 5, which shows the timing of the CQI reporting in LTE Rel-8. The UE receives the aperiodic CQI trigger on the PDCCH in the DL subframe #0, and performs the measurement based on the same subframe, e.g., the reference period for the CQI measurement is that same DL subframe #0. The aperiodic CQI report is sent on the PUSCH in the UL subframe #4.
Further, the eNodeB may allocate an UL transmission to the UE even though the UE does not have any data to transmit, or there may be DL data arrival when the TA timer of the UE has expired.
In the previous case the UE will transmit an empty BSR and padding. Although this particular type of allocation may not be likely to occur (e.g., if it does it may be due to an occurrence of a network error such as a loss of synchronization between the eNodeB and the UE), it could potentially occur whenever the UE is in an active state. It is implicitly defined in 3GPP TS 36.321 that the UE should then send the BSR.
In the former case, the DL data arrival PDCCH may be sent to allocate a dedicated preamble for RACH for the UE. This may occur when the UE's TA timer has expired and DL data has arrived at the eNode B. In this case it may not be critical to wait for a few milliseconds to the send the DL data arrival PDCCH.
One may consider that the eNB could send the UE an UL grant just in case the UE happens to have data in its buffer that the eNodeB is not aware of, e.g., polling the UE for data. This approach, however, has several drawbacks. For example, if the UL grant is made too early the eNodeB will simply receive an empty BSR, and if the UL grant is made too late then the UE will have already begun a data request, e.g., using a SR or the RACH procedure. Moreover, if the UE does have data to transmit it may, in any case, wait for the UL grant.
The fact that, as presently specified in Rel-8 of LTE, the UE must constantly monitor for UL grants is a clear disadvantage in those cases where monitoring for DL assignments is not necessary. One such scenario is in MBSFN subframes, where it has been agreed that DL assignments cannot be sent. Hence, forcing the UE to activate the RF circuitry to search for UL grants during those periods when the RF circuitry is not required for data transmission or reception results in the UE making unnecessary blind decodings of the PDCCH. The overall effect is an unnecessary increase in UE power consumption. Depending on the MBSFN configuration the number of such subframes can be significant, for example, up to six out of every 10 subframes may be configured as MBSFN subframes.
It should be further noted that the UE may not be able to measure CQI in a MBSFN or blank subframe, or during a measurement gap, since the reference signals may then have a different structure or may not be present. In such a case any aperiodic CQI triggered during such a subframe would be ambiguous and meaningless (as the UE 210 should perform the measurement for the aperiodic CQI in the DL subframe where the aperiodic CQI trigger is sent).